The present invention generally relates to DC chopper converters. More specifically, the invention relates to a DC chopper converter for controlling the amplitude and direction of a load current in a bidirectional control bridge of the type having four bridge arms, each arm having a switching element and a free-wheeling diode coupled in parallel, a load being coupled between the junctions of the arms such that an H-configuration is formed between two power supply terminals, switch elements that lie diagonally opposite each other across the bridge being periodically driven or operated so as to control the load current supplied to the load.
A DC chopper converter of the type described above is disclosed in European patent EP 0 024 300 and corresponding U.S. Pat. No. 4,314,325. The disclosures of these patents are fully incorporated herein by reference.
In the disclosed device, a circuit is provided for controlling the amplitude and direction of a load current in a bidirectional direct current control bridge of the type having four bridge arms, each arm having an electronic switching element electrically disposed therein, and a load, illustratively an ohmic-inductive load, connected to the junctures of the bridge arms. The load current is conducted to the load by applying energizing pulses to each of the switching elements in a pair of diagonally disposed bridge arms, the respective pulse energizing signals being out of phase with respect to one another. Load current is responsive to the duration of the interval during which the switching elements in the diagonal pair of bridge arms are simultaneously conducted.
Additionally, circuitry is provided for pulse energizing the electronic switching elements in the second diagonal pair of bridge arms, while inhibiting conduction of the pulse energizing signals to the first diagonal pair, in response to a change in polarity of a control signal. The amplitude of the control signal governs the pulse-width ratio of the pulse energizing signals, and therefore, the duration of the intervals of simultaneous conduction of the switching elements in a diagonal pair. Circuitry is provided for inhibiting the conduction of all pulse energizing signals for a predetermined interval of time in response to the change of polarity of the control signal.
U.S. Pat. No. 4,314,325 discloses other prior art devices for controlling load currents by the use of a four-arm bridge including a publication entitled "Siemens-Zeitschrift 43 (1969), No. 5 and U.S. Pat. No. 3,260,912.
The device stated to be disclosed in the Siemens-Zeitschrift publication comprises a basic four-arm bridge circuit containing electronic switching elements in which a first pair of electronic switching elements which are on diagonally disposed arms of the purchase circuit are periodically and simultaneously closed and opened while the remaining second pair of diagonally disposed switching elements remains open Reversal of the current through the load is achieved by simultaneously opening and closing the second pair of switching elements while the first pair remains opened. It is stated that this method of operating a four-arm bridge control circuit has the advantage of a linear relationship between the control voltage and the load voltage.
It is also stated that it is a disadvantage of the foregoing system that the polarity of the load voltage and the current which flows into the bridge changes during the opening intervals of the periodically operating electronic switching elements. This results because the inductive load component causes load current to remain flowing during intervals that the operated switching elements are open. Such current flows through free-wheeling diodes which are disposed and shunt across each such electronic switch. Such diode current flows back into the power supply, in a direction opposite to the current flow during the time that such switching elements are closed, load voltages are reversed concurrently with such bypass diode current. The effect o this operation is that the load will experience a current having a relatively large alternating current ripple component which produces additional heat loss in the load. Moreover, in situations where a motor is used as the load, the large alternating current component can create whining noises.
In U.S. Pat. No. 3,260,912, the teachings of which are fully incorporated herein, there is disclosed another DC chopper converter designed for reducing the amplitude of the alternating current component in the load current by the use of pulse-width control. In this system, a first pair of diagonally disposed electronic switching elements in the bridge are opened and closed during time intervals which are offset with respect to one another. Thus, during the open interval of each such electronic switching elements, the diagonally associated electronic switching elements remain closed. This offset driving arrangement provides an advantage over the above-described simultaneous driving arrangement because the load current which continues to flow after a particular switching element is opened, as a result of the inductive component of the load, does not flow back into the power supply, but circulates through the closed electronic switching element and a free-wheeling diode. This arrangement, therefore, produces after each cycle of switching elements closure which delivers to the load electrical energy from the power supply, a bypass phase which is distinguishable from the energy reversing backflow phase of the previously-discussed arrangement, which does not reverse the load voltage, but reduces it to zero. Thus, the alternating current ripple components in the motor load current have the same pulse frequency as in the first-mentioned arrangement, but only one-half of the magnitude. This arrangement, however, has the disadvantage of a non-linear relationship between the control voltage and the load voltage, particularly in the range of small control voltages. Thus, it is possible in situations where small control voltages are utilized, that the load voltage would approach zero before the control voltage approaches zero. This results from the fact that the conductive intervals of diagonally disposed electronic switching elements cannot overlap because a safety interval must be maintained to prevent electronic switching elements which are disposed on the same half of the bridge from being simultaneously conductive and causing shortcircuit conditions. Similarly, an overlap of the conductive intervals of diagonally opposite switching elements can occur when negative control voltages are utilized only when such control voltages exceed predetermined negative values.
It becomes apparent, therefore, that a bidirectional direct current control bridge, which is operated in accordance with the latter method described above, has a region of insensitivity in which the load voltage is zero for small control voltages. This operational characteristic is a significant disadvantage for most applications. Electronic switching elements of the second diagonal, which are conductive during the non-conductive interval of the first pair of electronic switching elements, do not carry any current. The current is conducted through the free-wheeling diodes which are poled for conduction in a direction which is opposite to the forward conduction of the respectively associated switching elements. This can cause, in some applications, damage to the switching elements.
In all of the arrangements discussed above, the zero crossings of the source current are followed to determine when to switch over from one driving bridge diagonal to the other. Zero crossings in each direction are detected by a pair of detectors that produce an output signal only when an input signal thereto exceeds a predetermined bias signal level. It can be appreciated that a gap can exist between the two bias voltages. As a result, there is a region of insensitivity to zero crossings about the zero crossing axis and this can lead to non-linearities in the load current during switch-over between bridge diagonals. Therefore, the low current can only be precisely set within a limited dynamic range. Certain applications, however, such as in the drive gradient coils for nuclear magnetic resonance tomography, require a greater dynamic range, for example from 2 mA through 200 A, in which the load currents must be precisely controlled.